Loudspeakers or speakers covert an electrical impulse into a mechanical impulse which produces sound, usually by way of the use of electromagnetism which moves a cone. For purposes of this disclosure, a “loudspeaker” is defined as an electro-acoustic transducer which converts an electrical signal into audio output. Such devices are integral parts of every common audio system. This process involves many difficulties and has proven to be the most problematic of the steps to reproduce sound. As a result, loudspeakers are almost always the limiting element in the fidelity of the acoustics of the reproduced sound in home, theater, or in many entertain systems. The other elements in sound reproduction are mostly electronic which are highly advanced and developed.
Ideally, a loudspeaker should create a sound field proportional to the electric signal of the amplifier. Due to the physics of sound radiation, the output is almost always less than ideal particularly in the low frequency region. In general, the common loudspeaker may be split into two parts: an electromechanical and a mechanical-acoustical part. The latter has a diaphragm, the vibration of which creates sound pressure. One of the greatest difficulties in the conversion of electrical into acoustical energy has been the realization of a prescribed (mostly flat) frequency response in a certain (mostly large, broadband) frequency range. The broadband frequency range, for purposes of this disclosure, is between 20 and 20,000 Hz (Hertz).
An unenclosed loudspeaker radiates sound as an acoustic “dipole”. This gives rise to a poor low frequency or bass response since sound from the back of the diaphragm cancels sound from the front. For purposes of this disclosure, low frequency or bass response refers to sound less than 200 Hz, 100 Hz, or 80 Hz. The sound also radiates highly directionally. To avoid these problems, the loudspeaker can be mounted in an infinite baffle, in which case it radiates into the “half space” in front of the baffle as a monopole. Even with infinite baffles, loudspeaker radiation efficiency lessens considerably at low frequencies with a simple baffle board. To deal with this impracticality, the infinite baffle is “folded” around the back of the loudspeaker, forming an ‘infinite baffle’ enclosure (a closed box). However, this does not solve the problem of poor bass response.
Even with a good enclosure a single loudspeaker can not be expected to deliver optimally balanced sound over the entire audible frequency range. The requirements of producing adequate acoustic output at both low and high frequencies are mutually incompatible. In the high frequency range, the driver needs to be light and small to be able to respond rapidly to applied signal. Such high frequency speakers are known as tweeters. On the other hand, a bass speaker should be large to efficiently match the impedance to air. Such speakers, called, woofers, must also be driven with more power to drive a larger mass. Additionally, due to human ear's low response to bass, more acoustic power must be supplied in the bass or low frequency range. Sometimes, a third, mid-range speaker is also used to achieve a smooth frequency response.
Referring now more specifically to the low frequency response, a subwoofer is a woofer driver used only for the lowest part of the audio frequency range such as below 200 Hz (e.g., consumer systems), below 100 Hz (e.g., professional live sound), or below 80 Hz (as in “THX” approved systems known in the art). Because the intended range of frequencies is limited, subwoofer system design usually has a single driver enclosed in a suitable enclosure. Sound in this frequency range can easily bend around corners by diffraction (as low frequencies are “non-directional”), so the speaker aperture does not have to face the audience, and subwoofers can be mounted in the bottom of the enclosure facing the floor for convenience. To accurately reproduce very low bass notes without unwanted resonances (typically from cabinet panels), subwoofer systems must be solidly constructed and properly braced; good speakers are typically quite heavy. Many subwoofer systems include power amplifiers and electronic sub-filters, with additional controls relevant to low-frequency reproduction. These alternatives are known as “active” or “powered” subwoofers. Active subwoofers, like active monitors, have built-in power amplification to boost low frequency sound. In contrast, “passive” subwoofers require external amplification.
The loudspeaker, which generates acoustic pressure, has an internal source impedance and drives an external load impedance. The ambient air medium is the ultimate coupling load, which presents a low impedance because of its low density. The source impedance of any loudspeaker, on the other hand, is high (compared to the impedance of ambient air), so there will be a considerable mismatch between the source and the load. The result is that most of the energy put into a direct radiating loudspeaker will not be released into the air, but will be converted to heat in the voice coil and mechanical resistances in the unit. The problem is worse at low frequencies where the size of the source is small compared to a wavelength. The source pushes the medium away. At higher frequencies, the radiation from the source is in the form of plane waves that do not spread out. The load, as seen from the driver, is at its highest, and the system is as efficient as it can be.
When the loudspeaker diaphragm vibrates, pressure waves are created in front, which creates the sound we hear. Coupling the motion of the diaphragm to the air properly is difficult due to the very different densities of the vibrating diaphragm and air. This can be viewed as an impedance mismatch. It is known that sound travels better in high density materials than in low density materials, and in a speaker system, the diaphragm is the high density (high impedance) medium and air is the low density (low impedance) medium. The horn assists the solid-air impedance transformation by acting as an intermediate transition medium. In other words, it creates a higher acoustic impedance for the transducer to work into, thus allowing more power to be transferred to the air.
Consumer electronic devices, such as cell phones, tablets, and the like with more features and capabilities are ubiquitous and are positioning to become entertainment centers. However, they also exhibit severe audio deficiencies as mentioned above and pose many additional challenges to maintain the acoustic performance as enclosed acoustic volume size, power and membrane size are reduced significantly. Due to the smaller size of the speaker used in such devices, the low frequency response is severely affected. For example, as the size of the cell phone decreases, the volume of air behind the diaphragm is reduced. This small amount of volume behind the speaker limits the range of motion of the diaphragm. The speaker does not produce enough force to compress the air beyond a certain point, hence causing the air to push back. This reduces the displacement of the speaker diaphragm, which in turn lowers the output. Thus, low frequencies are affected the most by this phenomenon as the diaphragm moves with the largest amount of displacement at these frequencies. Consequently, the frequency response usually rolls off faster at low frequencies (<300 Hz).
A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location. The medium is simply the material through which the disturbance is moving. In solids, sound waves travel in the form of the vibration or wave of molecules produced when an object moves or vibrates through a medium from one location to another. When an object moves or vibrates, the molecules around the object also vibrate, producing sound. Sound can travel through any medium except vacuum.
Sound fields radiated from loudspeakers can be divided into distinguishable regions. Two of which are the geometrical near field and the far field. Close to the source (the near field), for some fixed angle θ, the sound pressure falls off rapidly, p∝1/r^2. Thus in the near field, the sound pressure level decrease by 12 dB for each doubling of distance r. In the far field, the sound pressure levels decrease monotonically at a rate of 6 dB for each doubling of the distance from the source and are characterized by the criteria given below:r>>λ/(2π),r>>a,r>>πa^2/(2λ),where the inequality represents a factor of 3 or greater, r is distance to the source, a is the characteristic source dimension and λ is the wavelength of radiated sound. Thus, it is advantageous to design loudspeakers according to a far field criterion.
The most commonly used far field reference distance for loudspeaker SPL specifications is 1 meter (or 3.28 feet). Sound field of loudspeakers must be measured at a distance beyond which the shape of the radiation pattern remains unchanged as the changes are caused by path length differences to different points on the surface of the device. For relatively smaller loudspeakers sound field might possibly be measured at 1 meter, but for larger loudspeakers it needs a different far field measurement scheme. For large devices, the beginning of the far-field must be determined, marking the minimum distance at which radiation parameters can be measured. The resultant data can then be referenced back to the 1 meter reference distance using the inverse-square law. This calculated 1 meter response can then be extrapolated to further distances with acceptable error.
Sound-absorbing materials such as foams, fiberglass, absorbent panels, etc. are commonly used in various industries and buildings to reduce noise for which the sound waves are reflected, absorbed and transmitted when they hit a hard surface. A commonly used term to define and evaluate sound absorption is the sound absorption coefficient. The sound absorption coefficient is a measure of the proportion of the sound striking a surface, which is absorbed by that surface, and is usually given for a particular frequency. Thus, a surface which would absorb 100% of the incident sound would have a sound absorption coefficient of 1.00, while a surface which absorbs 35% of the sound, and reflects 65% of it, would have a sound absorption coefficient of 0.35. Materials which are dense and have smooth surfaces, such as glass, have small absorption coefficient, whereas porous-type materials, such as glass wool or fiberglass blankets, that contain networks of interconnected cavities tend to scatter the sound energy and tend to trap it. Therefore, there is greater interaction at the surface of such materials and more opportunities during these scattering reflections for the sound wave to lose energy to the material. Consequently, these materials possess relatively larger sound absorption coefficients in the mid to high frequency range, i.e. above 500 Hz.
A way of increasing the fidelity of acoustic reproduction of sound has long been desired. While sound quality does continue to improve, the efforts in increasing fidelity in far-field applications especially has largely stalled.